In typical sheet-fed offset printing machines, the printing ink is conveyed from a duct cooperating with a duct roller to a printing plate. More specifically, the ink ultimately is conveyed to the printing plate via a lifting roller and a number of additional inking rollers. Ordinarily, a number of ink-metering components, or elements, are disposed in the lower part of the duct in order to control the amount of ink delivered to the inking roller. These elements allow zonal ink-metering transverse to the direction of printing, i.e., across the duct roller, and as required by the printing plate. Typically, these elements are positioned in close proximity to the duct roller so as to produce an ink layer of defined thickness on the roller when it rotates. The thickness of the ink layer depends on the distance, or gap, between the ink-metering elements and the duct roller.
The ink-metering elements can be in the form of a slide with a movable tip (U.S. Pat. No. 4,711,175 corresponding to German Patent No. DE 3 503 736 C1), a blade-like ink-metering attachment integrally formed on a pivoting arm (U.S. Pat. No. 4,392,431, corresponding to German Patent No. DE 3 033 995 C2) or a duct blade divided into tongues the width of an inking zone. Preferably, each ink-metering element is associated with a remote-control adjusting drive, so that the positions of the ink-metering elements relative to the roller in all of the inking devices can be remotely adjusted from a central location.
The structure and operation of one such remotely controllable ink-metering system is shown in German patent No. DE 3 914 831 A1. In this type of system, it is necessary to periodically position the ink-metering elements so that they touch the duct roller (a position known as zero inking). This is done so that a reference position can be established by detecting the position while an element is in contact with the roller (i.e., at zero position). The reference position (in actuality a value such as an analog signal representative thereof) is then utilized for subsequent remote-controlled positioning operations.
These zeroing operations can be performed manually by a printer, who adjusts each individual ink-metering element of an ink-metering system to the zero inking (contact) position. When the desired position is achieved, the printer actuates a switch or other actuating device, whereupon a value representative of the position is stored in the remote-control system for use as the zero reference position.
As can be readily appreciated, such a manual procedure is time-consuming if there are a number of ink-metering elements (for example fifty or so) on a duct roller and a number of printing units in a printing machine. One attempt to simplify the zero positioning of the elements has been proposed in German Patent No. DE 3 914 831 A1, which provides sensors on the individual ink-metering elements. These sensors provide a distance signal corresponding to the position of the ink-metering element relative to the duct roller. At the moment that one of the aforementioned sensors, which operate without contact, ceases to deliver a signal varying in time as the ink-metering element approaches the roller, the ink-metering element is determined to be touching the duct roller (zero inking) and this precise sensor position is stored and used as the zero position. Alternatively, a separate signal, such as a voltage which varies as a function of the distance between the element and the roller, can be stored at the moment of contact as a characteristic voltage associated with the zero position. Thus, this method and system is able to automatically store the values representative of the zero positions. Although this method works efficiently, one of the major drawbacks to this system is that the sensors required for working the method greatly increase the cost of the ink-metering system.
German Patent No. DE 3 727 656 A1 discloses an ink-metering device comprising a duct with a duct blade divided into tongues the width of a zone and a duct roller electrically insulated from the duct and duct blade. Each individual tongue of the duct blade (the ink-metering element) essentially constitutes a switch which can be electrically interrogated; it becomes closed at the exact time that the ink-metering element is adjacent the duct roller. While this system also allows automated zero positioning, a disadvantage of this device is that the aforementioned principle is impracticable in situations where ink having high electric conductivity (for example, an ink containing metal) is used. A duct containing ink of this kind has to be emptied and cleaned before this type of zeroing can be performed.